Inhibiting corrosion



March l', 1960 G. A. MARSH ETAL INHIBITING coRRosIoN Filed Dec. 23, 1957nmkdoo wm wm C# (SBHONIOHDIN V* NOISOHHOQ IN VENTOR.

GLEN/v A MARSH BY EDWARD soHAQcHL ATTORNEY United States PatentINHIBITIN G CORROSION Glenn A. Marsh and Edward Schaschl, Crystal Lake,Ill., assignors to The Pure Oil Company, Chicago, Ill., a corporation ofOhio Y Application December `23, 1957, Serial No. 704,818

7 claims. (c1. V24m- 147) This invention relates to cathodic protectionof ferrous metal structures submerged under water or buried underground,and is more particularly directed to a method of treating suchstructures prior to submerging or burying them in order to reduce theamount of current necessary to give adequate cathodic protection againstcorrosion.

It is common practice to apply cathodic protection to :structures suchas pipe lines buried underground or submerged under water, as well as tostructures such as drilling platforms erected in water and ships used insalt water. It is also common practice to wrap, coat or encaseferrous'metal structures which are buried or submerged in order toprevent corrosion. Such methods are expensive. Where coating or wrappingis not practiced, the amount of current required to aiford necessarycathodic protection is usually prohibitive.

We have discovered an inexpensive method `of treating such structuresprior to burying underground or sub- `merging in water which isinexpensive and enables adequate cathodic protection to be obtained withapproximately 2-20% of the currentwhich is ordinarily required foruncoated structures.

Our invention resides in coating the surface with a slution of an alkalimetal hexametaphosphate, drying the coating, and then applying a secondcoating of a solution of an alkali metal chromate or dichromate and aphosphoric acid, and drying the second coating prior to submerging orburying the pipe.

It is 4an object of our invention to provide an -inexpensive method forprotecting ferrous metal structures buried underground or submergedunder water against corrosion. It is another object of our invention toprovide an inexpensive coating on ferrous metal structures which willenable buried or submerged structures to be cathodically protectedagainst corrosion by a small amount of current. Other objects of theinvention will become manifest from the following description and theaccompanying drawing, of which the single ligure is a graph showing rateof corrosion of test specimens when protected in accordance with thisinvention as compared with specimens which are not so protected.

In accordance with our invention, the structure to be protected, such aspipe or piling, is wire-brushedor sanded to remove loose rust, and thentiegreased by scrubbing with carbon tetrachloride or equivalent greasesolvent. The clean structure is then coated with a solution, preferablyfreshly prepared, of an alkali metal hexametaphosphate, preferablysodium hexametaphosphate. A solution of any desired concentration may beused but we prefer Va solution `of about 5-20% by weight sodiumhexametaphosphate. The solution is applied to the structure by brushing,spraying, dipping, or any other suitable manner. Where the coating isperformed by dipping or immersing, the specimen may be left in thesolution for a period of l-100 minutes at a temperature `of about 40l20F. We prefer to maintain the solution at about 60-90" F. and allow thestructure to rerice main immersed for about 5-15 minutes, after which itis dried in air and then given a second coating with a solution of analkali metal chromate or dichromate, such as sodium or potassiumdichromate, containing a phosphoric acid, such as meta, ortho, orpyro-phosphoric acid. We prefer to use a dilute, aqueous solutioncontaining about 0.5-1% of sodium dichromate and inthe neighborhood ofabout 0.l1% of` phosphoric acid. Dilute solutions should be used so thatwhen the coating is dried it will be very thin, weighing less than 1gram per sq. ft. In coating with the dichromate-phosphoric acid solutionthe temperature of the solution should be maintained at about 40-120 F.,and where immersion is used, the period of immersion will be about 1-100minutes. Preferred conditions of immersion are l20 F. fora period of5-10 minutes.

Where the structure to be protected is one which is partially corrodedor rusted, it has been found expedient to repeat the hexametaphosphatecoating after coating with the dichromate-phosphoric acid solution, anddrying, and to also repeat the dichromate-phosphoric acid coating afterthe second hexametaphosphate coating and drying. Where the structure tobe protected is clean, only a `single coating with the hexametaphosphateand with the dichromate-phosphoric acid solution is required,

In order to demonstrate the invention, an ordinary rusted steel specimenwas wire-brushed to remove loose rust and then degreased by scrubbingwith carbon tetrachloride. The degreased specimen was immersed for 15minutes at room temperature in a freshly prepared 20% solution ofhexametaphosphate, after which it was removed, drained and allowed todry in air. After the specimen was dried it was immersed for l0 minutesin an aqueous solution at F., containing 0.67% by weight of sodium`dichromate and 0.5% by weight of 85% phosphoric acid. The specimen wasremoved from the second solution, drained, and dried in air. Thetreatment just described with the two solutions was repeatedon thespecimen, after which the specimen was immersed in aerated, agitatedwater and cathodic protection was applied at a current density of 3milliamperes per sq. ft. of surface area. No visible corrosion occurredover a period of a 30-day test period.

In order to further demonstrate the efcacy of the method constitutingour invention, a series of experiments were performed, the results ofwhich are graphically represented in the drawing. In these testscorrosion test- -probes of the type similar lto those sold by Labline,Inc. and disclosed in their Bulletins Nos. 52 and 5205, and claimed inour copending application Serial No. 528,032, led August 12, 1955 andalso in application of Lynn E. Ellison, Serial No. 631,697, led December3l, 1956, Patent No. 2,824,283, in which the test element was made ofmild steel, were used in the tests, and loss of thickness was readdirectly on the corrosion meter. The results in these tests are shown inthe drawing by plotting a loss in thickness in micro-inches againsttime. The slope of the curve at any point is the corrosion 4rate at thatparticular time. Curve A represents the results obtained using a probewhich had been coated in the manner just described in connection withthe previous example, but to which no cathodic protection was applied.Curve B shows the results obtained with a probe which had not beencoated but to which a cathodic protection of l m.a./ sq. ft. currentdensity had been applied. `Curve C shows the results obtained when theprobe was coated in the manner described in connection with the previousexample and to which cathodic protection current had been applied lat acurrent density of l ma./ sq. ft. In this test the current wasinterrupted for one hour during each 24 hours. In a similar test,represented by curve D, the current was applied continuously withoutinterruption. The results 3 as depicted in the drawing show that notonly does the combination of coating and low :current-density eathodicprotection aiord substantially complete protection against corrosion,but, it further shows that interruption of the current does not have aserious detrimental effect on the protection afforded. Maximumprotection is obtained, however, when the current is continuous.

A further series of tests was run to `determine whether other solutionswhen used in conjunction with lov/density cathodie current would protectrusted steel against corrosion. The vario-us solutions used in lthesetests and the 4combinations in which the solutions were used for coatingthe probes are given in the following Tables I and II.

TABLE I Solutions used in tests Identication Composition 50% EQOA-50%Formula 30 alcohol. 10% NaOH.

5% Chromie Acid. 10% Chromic Acid.

Cone. HNOa. 30% HNOQ. 2% Versene Fe3.

5% Sodium Hexametaphosphate. 20% Sodium Hexametaphosphate. StraightHaPO4. 50-50 HgPOi-l-Formula 30 alcohol.

20% Sodium Hydroxide. y A 20% Sodium Hydroxide saturated in SodiumNitrite. Water Glass. Water Glass 90%-l-Calcium Chloride 10% plus 500 g.

dist. water. Saturated sodium chromate. 50-50 Satd. potassiumdichromate-H3P04.

Saturated potassium dichromate solution, 500 cc.l1

cc. sulfuric acid. 50-50 Phosphoric acid-l-distilled water plus 10 g.iron filings. i

50-50 H3PO4+Formula 30 alcohol (added 10 g. manganous sulfate).

50-50 Phosphoric acid-l-water solution (to 200 cc. o this added zincgranules until just saturated with zinc, then added 20 cc. phosphoricacid).

350 g.850 g. Phosphoric acid-l-HZO (added iron lings in excess, afterbubbles stopped; filtered through coarse lter paper; added 100 cc. ofHiPOl; then added 10 cc. of manganous chloride).

250 g.-250 g. HgPOl-l-water (added zinc granules in excess, afterbubbles stopped; poured oi the solution leaving zinc behind. Then added50 ce. H3PO4. Then added l cc. manganous chloride).

Saturated Calcium Chloride-l5% HgPOl.

% Tri-Sodium Phosphate.

% Potassium Permanganate.

5% Ammonium Persulfate.

50-50 (20% Sodium Hydroxide saturated with Sodium Nitrite)water glass.{50% Water Glass.

50% NaOH. 50% Water Glass-5% (20% NaOH) 45% water. 5% Water Glass-95%(20% NaOH). 5% Water Glass, 45% (20% NaOH) 50% water. Calcium Chloride.10% Copper Sulfate. Mercurio Nitrate. 5% Sodium Arsenite. SaturatedSodium Chromate. Saturated sodium bicarbonate. 0.67% Sodium Dichromate.0.5% Phosphoric Acid. 98.83% Water. 0.2% Sodium Nitrite. 0.2% DiammoniumHydrogen Phosphate. 99.6% Water. 0.5 Sodium Nitrite. 0.5% DiammoniumHydrogen Phosphate. 99.0% Water. 0.7% Sodium Nitrite. 0.7% DiammoniumHydrogen Phosphate. 98.6% Water. 0.5% Sodium Nitrite. 0.25% DisodiumPhosphate.

% Monosodium Phosphate.

99.00% Water.

0.5% Sodium Nitrite.

0.37% Disodium Phosphate.

0.13% Monosodium Phosphate.

99.00% Water.

Saturated Trisodium phosphate` Sodium Nitrite 0.5%.

0.5% Diammonium Phosphate.

{0.5% Sodium Nitrite.

0.5% Diammonium Phosphate. 99.00% Water.

0.5% Sodium Nitrite.

0.5% Disodium Phosphate. 0.1% Monosodium Phosphate. 99.00% Water.

TABLE @continued Identcation Composition 0.5% Sodium N itrlte. 41 0.25%Disodium Phosphate.

.25% Water. .5% Sodium Nitrite. .1% Dlsodium Phosphate. 0.4% MonosodiumPhosphate. 99.00% Water. 43 {1% Dritilm.

"""""""" 99%Hexane.

Silicone (Mfg. by'Dow Corning). Silicone (Mfg. by G.E.)

Gora

TABLE II Solution applied as coating and order in which applied to testspecimen Second First Coating Coating Coating 'In no case were thesolutions effective in affording p rotection except in the two tests inwhich the probe was initially coated with the sodium hexametaphosphatesolution and then coated with'the sodium dichromatephos phoric acidsolution.

Our invention is equally eiective in protecting khotv,rolled andcold'rolled steel and in protecting other ferrous metal alloys.

The coatings are veasy and inexpensive to apply, it 'being necessaryonly to coat the entire surface to beprotected and permit each coatingto dry before applying the .subsequent coating or before .mmersing orburying the structure. Whereas it is often necessary to use in theneighborhood of 30-50lma./sq. ft. current density to protect uncoatedstructures, we are able to fully protect structures which have beencoated `in accordance with our invention by the application of as littleas 1 ina/sq. ft. of current density. in applying our invention to astructure to be protected, We prefer after burying or submerging thestructure to initially apply a higher current density of the order ofl0-20 ma./sq. ft. and gradually decrease the current density to theminimum required to aiord protection as indicated by corrosionmetertests.

What is claimed is:

l. In a process for cathodically protecting ferrous metal structures,submerged in water or buried beneath the ground, against corrosion, thesteps of coating the bare metal structure with a solution of alkalimetal hexametaphosphate, drying the coating, then coating the structurewith a second solution containing a phosphoric acid and an alkali metalsalt of the group consisting of alkali metal chromates and dichromates,drying the coating before submerging or burying the structure,contacting said coating with the water or ground, and cathodicallyprotecting the structure by applying thereto a current having a densityequal to a small fraction of that required to protect the uncoatedstructure against corrosion.

2. Process in accordance with claim 1 in which the structure is cleanedand degreased prior to applying the solution of alkali metalhexametaphosphate thereto.

3. Process in accordance with claim 2 inV which the alkali metalhexametaphosphate is sodium hexametaphosphate.

4. Process in accordance with claim 3 in which the second solution is adilute solution of phosphoric acid and sodium dichromate.

5. Process in accordance with claim 4 in which the sodiumhexametaphosphate is an aqueous solution containing 5-20% of sodiumhexametaphosphate and the second solution is an aqueous solutioncontaining about 0.67% sodium dichromate and about 0.5% phosphoric acidby weight.

6. A process according to claim 1 in which the current density is about1 milli-ampere per square foot.

7. A process according to claim 1 in which the current density isinitially 10-20 milli-amperes per square foot and is gradually decreasedto the minimum current density required to afford substantially completecathodic protection as indicated by corrosion rate measurements.

Reereuces Cited in the le of this patent UNTED STATES PATENTS 2,163,984Pekovic June 27, 1939 2,337,856 Rice Dec. 28, 1943 2,499,261 RosenbloomFeb. 28, 1950 2,787,557 Christensen et al. Apr. 2, 1957

1. IN A PROCESS FOR CATHODICALLY PROTECTING FERROUS METAL STRUCTURES,SUBMERGED IN WATER OR BURIED BENEATH THE GROUND, AGAINST CORROSION, THESTEPS OF COATING THE BARE METAL STRUCTURE WITH A SOLUTION OF ALKALIMETAL HEXAMETAPHOSPHATE, DRYING THE COATING THEN COATING THE STRUCTUREWITH A SECOND SOLUTIOIN CONTAINING A PHOSPHORIC ACID AND AN ALKALI METALSALT OF THE GROUP CONSISTING OF ALKALI METAL CHROMATES AND DICHROMATES,DRYING THE COATING BEFORE SUBMERGING OR BURYING THE STRUCTURE,CONTACTING SAID COATING WITH THE WATER OR GROUND, AND CATHODICALLYPROTECTING THE STRUCTURE BY APPLYING THERETO A CURRENT HAVING A DENSITYEQUAL TO A SMALL FRACTION OF THAT REQUIRED TO PROTECT THE UNCOATEDSTRUCTURE AGAINST CORROSION.